The invention relates to sorting using a conveyor, particularly to an apparatus and method for sorting items using multiple carts traveling around a closed loop.
The postal system and high volume package shipping industry use tilt tray and cross-belt conveyor systems to sort bundles of letters and packages according to their respective destinations. Specialized sorters sort a bundle or package by destination zip code. During operation, an input stream of parcels is placed on a conveyor and sorted into multiple output streams. The conveyor sorts the packages by unloading them to either another appropriate conveyor or to an intermediate destination such as an unloading station. The unloading operation can be carried out with a tilt tray mechanism that tilts and ejects the package or bundle, or a cross-belt conveyor that unloads the item by means of a moving belt that conveys the item laterally.
Prior art tilt tray conveyor systems comprise a series of tilt tray carts linked together in a continuous loop. According to one known tilt tray conveyor system, the trays are secured to an endless drive chain, which pulls the trays around the loop. See Muller U.S. Pat. No. 3,662,874, issued May 16, 1972. According to another known tilt tray conveyor system known as the Mantissa Scorpion, linear induction motors (LIM""s) are disposed at intervals around the loop for acting on a horizontally or vertically disposed plate (drive element) on each cart. The frame of each cart is T-shaped with a single axle, so that each cart depends on an adjoining cart for support.
Prior art cross belt sorters similarly comprise an endless loop of carts. Items to be conveyed are deposited on the cross-belt, which replaces a tray as the carrier. During unloading, an electric motor drives the conveyor so that the item is moved off the conveyor surface to one side or the other. See, for example, U.S. Pat. No. 5,690,209, issued Nov. 25, 1997 and also European Patent Application 927,689, published Jul. 7, 1999.
FIG. 1 illustrates a conventional loop 5 of LIM-driven tilt tray carts 10 connected head to tail and mounted on an endless, generally oval-shaped rail 12. The continuous loop of carts creates significant inefficiencies in the conveyor system. First, the system""s strength depends literally on its weakest link. For example, if one cart 10 or its tray fails, the entire system must be stopped until the cart is repaired or replaced. Second, inefficient loading frequently occurs. The system may skip carts to maintain conveyor speed. This creates a situation in which empty carts are pulled around the loop, thereby resulting in wasted energy and system capacity. Additionally, some applications require large distances between input and output streams. Increased costs associated with longer cart chains may prohibit using a continuous chain conveyor system in a large loop.
Referring now to FIG. 2, transferring parcels between multiple loops 5A and 5B requires unloading the parcel from loop 5A and transferring it to the other loop 5B by a gravity slide 22 which feeds parcels to conventional conveyor belt 24. Belt 24 delivers the parcels to a powered induction station 26 which loads it onto a tray of a cart 10 in loop 5B. The potential for parcel damage occurs with each transfer to and from the carts 10. This manner of transfer between loop 5A and loop 5B introduces many opportunities for the item to be damaged because moving an item to or from trays involves subjecting the item to forceful impacts.
Inefficiencies caused by the method of locomotion also exist. According to another known conveyor design called the NovaSort, a product of Siemens ElectroCom, L.P., a train or segment of tilt tray carts connected end to end is drawn by a leading cart having an engine in the manner of a monorail. The lead cart draws power from a sliding electrical contact on the rail. This design suffers the customary drawbacks of systems that rely on sliding electrical contacts. In addition, the carts of each segment contain a solenoid that actuates the tilting mechanism on each cart, thus adding to the weight and complexity of the system.
Accordingly, a low-maintenance cart system is needed that reduces the potential for parcel damage created by cart transfers between loops.
A sorter conveyor system according to the invention includes at least one endless conveyor loop including a rail. One or more conveyor segments are mounted on the rail. Each segment is a series of cart units each having wheel structures mounted for rolling movement along the rail, a carrier such as a tray or cross-belt for carrying one or more items thereon, a selectively actuable mechanism for unloading the carrier, such as by tilting a carrier tray laterally in at least one direction to unload an item to an unloading station adjacent the conveyor loop, and a pivotable coupling mechanism for joining each cart unit in each series in a head to tail relationship. One or more drive elements are connected to one or more of the cart units and configured to permit the conveyor segment to be driven by a linear drive unit. A drive system is provided which includes a plurality of linear drive units, preferably linear induction motors (LIM""s) disposed at spaced positions along the conveyor loop for driving each of the drive elements of the cart units in each segment, such that each conveyor segment can each be driven independently of each other conveyor segment by selective actuation of the linear induction drive units. Other types of linear drive units, such as mechanical systems which directly pass momentum to the cart as it passes or systems which rely on forces other than magnetism, could also be used.
The first and last carts in each segments are connected to only one adjoining cart, that is, are not connected or adjacent to each other in a manner effective to form a continuous cart loop as in the prior art. Where the system has two or more cart segments, for example, selective control of the LIM""s can be used to move one segment independently of other segments on the same rail, but without need for an xe2x80x9cenginexe2x80x9d, i.e., a front or rear cart that pulls or pushes the series of carts in a manner analogous to a railroad train engine.
A linear drive unit as referred to herein means any form of conveyor drive, including both mechanical and linear induction, that exerts a force on a cart as it passes by, propelling the cart linearly (in the direction of the rail the cart is traveling on). The force may be exerted intermittently, as when a fin or plate on the cart passes by the linear drive unit, or continuously, as where the fin or plate spans multiple carts in the segment. In the alternative, spaced drive elements may be deployed on some carts and not others, such as on every other cart in the segment, as long as there are enough drive elements to keep the entire segment moving as required by the system design.
The invention further provides a method for sorting and conveying using a sorter conveyor system as described above. The method comprises the steps of moving the conveyor segment past a loading station, loading items onto the carriers of one or more of the carts as the carts pass the loading station, actuating the linear drive system to move the segment of carts past a row of unloading stations, and unloading items from the cart carriers to the unloading stations in accordance with a sorting scheme. Since the cart segment does not occupy the entire rail, the linear drive units may if desired be actuated only as a drive element of a cart is passing by. Similarly, two or more cart segments may be independently controlled on the same rail, for example, as where one is passing the loading station as the other is passing the unloading stations, after which the two cart segments exchange roles. These and other aspects of the invention are discussed in the detailed description which follows.